The present invention relates to a process for preparing high density polyethylene in the gas phase comprising contacting ethylene or a mixture comprising ethylene and one or more alpha-olefins with a chromium oxide catalyst supported on a granular or microspherical refractory oxide in a fluidised bed reactor in the presence of oxygen. The present invention further relates to a process for controlling the density of high density polyethylene prepared in the gas phase comprising contacting ethylene or a mixture comprising ethylene and one or more alpha-olefins with a chromium oxide catalyst supported on a granular or microspherical refractory oxide in a fluidised bed reactor in the presence of oxygen.
In two foreign references, Ermakov et al., “Transfer Processes During Polymerization of Ethylene on a Chromium Oxide Catalyst, II, The Role of Impurities in Transfer Reactions,” Kinetika i Kataliz (USSR), Vol. 10, No. 2 (1969) and Dahlig et al., Khimiya i Teknologiya Polimerov, No. 4, 23 (1961), the use of oxygen (and also carbon monoxide, acetylene, carbon dioxide, and hydrogen in the Ermakov reference) is disclosed as having the effect of lowering the molecular weight of polyethylene produced in a chromium based catalyzed polymerization of ethylene.
The prior art also includes some references disclosing the use of oxygen in the course of the gas phase polymerisation of ethylene.
U.S. Pat. No. 5,473,027 (Chevron) discloses a process for making polyethylene resin by reacting ethylene in the presence of a titanium modified chromium catalyst on a silica support in a gas-phase polymerization reactor and in the presence of a catalyst productivity reduction agent such as oxygen. The presence of oxygen allows to increase the melt flow ratio (MFR) of the resin which is particularly suitable for use in blow-molding applications.
It is thus known that high density polyethylene showing superior physico-mechanical properties (e.g. MFR, die swell, . . .) could be obtained by polymerising ethylene with a chromium oxide catalyst in a fluidised bed reactor in the presence of oxygen.
However, the Industry is continuously striving to find high density polyethylene which can be more easily processed and which exhibits still better product performances.